fix(datepicker): Parse date from $viewValue instead of $modelValue

[decafdennis]

This patch allows the use of datepicker in conjunction with an ngModelController that has custom formatters and parsers to translate between $modelValue and $viewValue.

In my particular use case, my model is a custom date value object. I'd like to bind it directly to the datepicker and handle the translation between the custom date value format and a Date object, with a formatter and a parser that's added to the ngModelController. This might be a somewhat obscure use, but it is valid and saves doing manual $scope.$watch's and so on.

Here's a demonstration of my use case: http://jsfiddle.net/dy8vvyey/7/

The datepicker directive, being the view, should really use the $viewValue, and it should be up to the user of the directive what the $modelValue looks like.

[wesleycho]

This makes a lot of sense - good catch!

Landed as 0ecf7fa.